1. Field of the Invention
The present invention relates to a tunneling magnetoresistance (TMR) transducer and a method for manufacturing the TMR transducer.
2. Description of the Related Art
As magnetic storage apparatuses have been developed in size and capacity, highly sensitive magnetoresistive (MR) sensors (heads) have been put in practical use (see: Robert P. Hunt, xe2x80x9cA Magnetoresistive Readout Transducerxe2x80x9d, IEEE Trans. on Magnetics, Vol. MAG-7, No. 1, pp.150-154, March 1971). Since use is made of the anisotropy magnetoresistance effect of NiFe alloy, these MR heads are called AMR heads.
Recently, more highly sensitive giant magnetoresistance (GMR) sensors (heads) have also been developed in order to achieve higher areal recording density (see: Ching Tsang et al., xe2x80x9cDesign, Fabrication and Testing of Spin-Valve Read Heads for High Density Recordingxe2x80x9d, IEEE Trans. on Magnetics, Vol. 30, No. 6, pp. 3801-3806, November 1994). A typical GMR head is constructed by a free ferromagnetic layer, a pinned ferromagnetic layer and a non-magnetic conductive layer sandwiched by the free ferromagnetic layer and the pinned ferromagnetic layer. In the GMR head, the resultant response is given by a cosine of an angle between the magnetization directions of the free ferromagnetic layer and the pinned ferromagnetic layer.
The GMR head as well as the MR head also serves as a temperature sensor. In other words, the resistance of the head is susceptible to the temperature thereof. Therefore, if the GMR head is applied to a read head of a magnetic storage apparatus for a hard magnetic disk, the GMR head is in contact with the hard magnetic disk, thereby greatly increasing the temperature of the GMR head, which can cause a problem known as a thermal asperity problem.
Thermal asperity per se is discussed in F. W. Gorter et al., xe2x80x9cMagnetoresistive Reading of Informationxe2x80x9d, IEEE Trans. on Magnetics, Vol. MAG-10, pp. 899-902, 1974 and R. D. Heristead, xe2x80x9cAnalysis of Thermal Noise Spike Cancellationxe2x80x9d, IEEE Trans. on Magnetics, Vol, MAG-11, No. 5, pp. 1224-1226, September 1975.
In particular, if the gap between the GMR head and a magnetic medium becomes less than about 40 nm, the thermal asperity problem becomes serious. In order to avoid the thermal asperity problem, the surface of a magnetic medium needs to be specially smoothed or a complex compensation circuit is required, which increases the manufacturing cost.
On the other hand, if the GMR head is applied to a magnetic tape apparatus for a soft magnetic disk or a floppy disk, the GMR head is often in contact with the floppy disk even if an air bearing is introduced. Therefore, it is impossible to apply the GMR head as well as the MR head to such a magnetic tape apparatus.
Additionally, use of a tunneling magnetoresistance (TMR) transducer as a read head has been investigated. A typical TMR transducer is constructed by a free feromagnetic layer, a pinned ferromagnetic layer and a tunnel barrier layer made of non-magnetic insulating material sandwiched by the free ferromagnetic layer and the pinned ferromagnetic layer.
In a first prior art TMR transducer, alumina is grown on an aluminum layer by an oxygen glow discharging process to obtain a tunnel barrier layer having a high TMR ratio of 18 percent (see: Jagadeesh S. Moodera et al., xe2x80x9cFerromagnetic-insulator-ferromagnetic tunneling: Spin-dependent tunneling and large magnetoresistance in trilayer junctionsxe2x80x9d, Journal of Applied Physics, Vol. 79(8), pp. 4724-4729, April 1996). In more detail, a ferromagnetic layer made of CoFe is deposited on a glass substrate by a vacuum evaporation process, and then, an about 1.2 to 2.0 nm thick Al layer is deposited on the ferromagnetic layer also by a vacuum evaporation process. Next, the surface of the Al layer is exposed to oxygen and oxygen glow discharging process to obtain an alumina layer as the tunnel barrier layer.
In the first prior art TMR transducer, however, the oxygen glow discharging process produces oxygen ions and active oxygen such as radical oxygen, which makes the control of the thickness of the tunnel barrier layer difficult. Also, the tunnel barrier layer is contaminated by such oxygen, therefore degrading the quality of the TMR tranducer.
A second prior art TMR transducer has suggested that an Al layer be exposed to atmospheric air so as to form an alumina layer as a tunnel barrier layer (see: JP-A-63254, JP-A-6-244477, JP-A-8-70148, JP-A-8-70149, JP-A-8-316548 and N. Tezuka et al., xe2x80x9cRelationship between the Barrier and Magnetoresistance Effect in Ferromagnetic Tunneling Junctionsxe2x80x9d, Japan Applied Magnetics Proceeding, Vol, 21, No. 4-2, pp. 493-496, 1997).
In the second prior art TMR transducer, however, pin holes may be generated in the tunnel barrier layer by particles in the air, and the tunnel barrier layer is also contaminated by water, carbon oxide and nitrogen oxide in the air, thereby degrading the quality of the TMR transducer.
A third prior art transducer has suggested a TMR transducer which is not dependent upon the temperature (see: S. Kumagai et al., xe2x80x9cFerromagnetic Tunneling Magnetoresistance Effect for NiFe/Co/Al2O3/Co/NiFe/FeNn Junctionsxe2x80x9d, Japan Applied Magnetics Proceedings, Vol. 22, No. 4-2, pp. 561-564, 1998).
In the third prior art TMR transducer, the TMR ratio and saturated resistance are not dependent upon the annealing temperature; however, there is no discussion on the dependence of the TMR ratio on the temperature of the TMR transducer which in practical use. Additionally, the resistance of the third prior art TMR transducer is so high that it is impossible to apply this TMR transducer to a read head of a magnetic storage apparatus.
A fourth prior art TMR transducer has suggested an inert metal layer as a tunnel barrier layer in order to suppress the occurrence of pin holes therein, thus obtaining a high TMR ratio (see JP-A-10-208218).
A fifth prior art TMR transducer has suggested that a lower ferromagnetic layer be directly connected to a substrate so as to suppress the occurrence of pin holes in a tunnel barrier layer, thus obtaining a high TMR ratio (see JP-A-10-93159.
The fourth and fifth prior art TMR transducers, however, do not discuss the temperature dependence of the TMR transducer.
It is an object of the present invention to provide a TMR transducer capable of suppressing the thermal asperity problem.
Another object is to provide a method for manufacturing such a TMR transducer.
According to the present invention, in a tunneling magnetoresistance transducer including first and second ferromagnetic layers and a tunnel barrier layer made of insulating material sandwiched by the first and second ferromagnetic layers, the resistance of the tunnel barrier layer remains essentially constant independent of the temperature of the transducer.